Underclothes are the clothes you wear right against your skin, under everything else — underpants, bras, vests, undershirts. The word is always plural: you say 'my underclothes are wet', never 'an underclothe'. To count them you need another word: 'a pair of underpants', 'two items of underwear'. In real speech, most people say 'underwear' — that is by far the commonest word today, and it works as a singular uncountable noun, so 'my underwear is in the wash' is fine. 'Underclothes' sounds slightly older and slightly more careful, the kind of word a parent packing a suitcase might use, or a hospital form. 'Undies' is the playful, casual version; 'lingerie' (from French) means women's underwear that is meant to look beautiful, and sounds either fancy or suggestive depending on who is speaking; 'smalls' and 'underthings' are older British words you may meet in books. Note also 'underclothing', which is uncountable and even more formal. Because underwear is a slightly private topic in English-speaking cultures, people often use these words in a quick, matter-of-fact way rather than dwelling on them.

內衣褲（貼身穿在裡面的衣物）。台灣日常最常說「內衣」「內褲」分開講，英文的 underclothes／underwear 是把兩者一起概括的字，而且永遠是複數形。